Screw Propeller. 25
the scum, in order to leave the casting sound. Wherever the
molten metal is to touch the iron plates the latter should be
washed with loam.
Foundry Pits.—It should be understood that the floor we
have spoken of in the last example is not strictly the foundry
floor, but that of a pit deep enough to hold the whole mould.
Important castings like the one last considered, and especially
upright ones, are always thus treated, and after the mould has
been finished the space left in the pit is filled in and rammed so
as to bed the mould tightly against the sides of the pit, and thus
resist the pressure of the metal on casting.
A Screw Propeller can be best moulded in loam, a pattern
being provided for the centre boss. Referring to Fig. 35, a board
A centred on the vertical spindle, and balanced by means of a
small weight, is revolved so as to travel along the incline B c,
which is only a template curved so as to have D as its centre, and
forming part of a screw of the same pitch as the propeller. It is
very clear then, that by backing up the surface B c with loam, we
shall obtain a screw surface the same as that of the propeller blade
required. The next thing is to mark out the shape of the blade,
shewn in dotted lines. On the blade thus marked out, dried and
blackwashed, we now lay strips of wood, as shewn atŁ, Fig. 36,
representing the thickness of the propeller blade, and the surface
is then covered with loam up to this thickness, smoothed off, and
again dried and blackwashed, Now completely coyer with loam,
and so form top mould, which in its turn is taken away and dried.
The thickness piece being removed, the blade is completely
moulded, and this may be repeated for the other blades. Setting
all the lower moulds then in position on the floor, the bottom half
of boss pattern is applied (Fig. 37), and, being filled round with
dry sand at E E, the top half is treated similarly. Lastly, the
mould is completed by the addition of a core for the central hole,
and of the top box, and the whole has the appearance of Fig. 38.
A large Fly-wheel may be moulded without the necessity
of making a pattern for the whole of it A coke bed is first
formed on the floor for the purpose of venting, and a centre is
sunk for the spindle A, Fig. 39. Then the core box in Fig. 40 is
taken, which is formed so that a certain number of cores made